ci: pass ghprPullId in make-containerized-build job

In case the environment variable is not set at all (as opposed to
empty), teh following error is reported while running the job:

    groovy.lang.MissingPropertyException: No such property: ghprbPullId for class: groovy.lang.Binding

Signed-off-by: Niels de Vos <ndevos@redhat.com>

make-containerized-build.yaml

@@ -11,6 +11,10 @@
             - ci/centos
       script-path: make-containerized-build.groovy
       lightweight-checkout: true
+    parameters:
+      - string:
+          name: ghprbPullId
+          default: ''
     triggers:
       - github-pull-request:
           admin-list: